Motor and turbo-molecular pump

ABSTRACT

A compact turbo-molecular pump having a high depressurizing capability. A motor for rotating a rotor vane includes an air bearing. The air bearing has a rotary cylinder and a fixed surface surrounding the rotary cylinder. The material of the rotary cylinder has a coefficient of thermal expansion that is smaller than that of the material of the fixed surface. Thus, change in the dimensions of the rotary cylinder is smaller than that of the fixed surface even if the temperature of the air bearing rises during operation of the pump. Thus, the rotary cylinder avoids contact with the fixed surface.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a motor and a turbo-molecularpump.

[0002] A turbo-molecular pump produces an ultra-high vacuum state and isemployed in, for example, semiconductor fabrication related apparatuses(e.g., sputtering apparatuses, chemical vapor deposition (CVD)apparatuses, and etching apparatuses) and measuring apparatuses (e.g.,electron microscopes, surface analysis apparatuses, and environmenttesting apparatuses). A typical turbo-molecular pump includes a motorand a plurality of rotor vanes rotated by the motor. The rotor vanes arerotated to produce a molecular flow and discharge gases. This causes anultra-high vacuum state in the interior of the apparatus connected tothe turbo-molecular pump.

[0003] The motor has a rotary shaft that is rotated at a high speed toproduce the ultra-high vacuum state. The bearing that supports therotary shaft must thus be capable of high speed rotation. A ballbearing, which requires lubricating oil, is not appropriate for suchapplication. This is because the vapor pressure of the lubricating oil,although low, hinders depressurization to the ultra-vacuum state by theturbo-molecular pump. Further, vaporized lubricating oil contaminatesvacuum chambers.

[0004] Japanese Unexamined Utility Model Publication No. 63-14894 andJapanese Unexamined Patent Publication No. 2-16389 describes aturbo-molecular pump that does not use lubricating oil. Thisturbo-molecular pump employs non-contact bearings, such as air bearingsor magnetic bearings.

[0005] A kinetic air bearing is one example of an air bearing. This airbearing has a fixed cylinder and a rotatable cylinder, which is arrangedin the fixed cylinder. A bearing area and a seal area are defined on theouter surface of the rotatable cylinder. A plurality of dynamic pressuregrooves extend along the bearing area. A predetermined clearance isprovided between the outer surface of the rotatable cylinder and theinner surface of the fixed cylinder.

[0006] One end of the two cylinders is exposed to a predetermined vacuumatmosphere. Thus, the seal area is located near that end of therotatable cylinder to prevent gases from moving between a compressed gaslayer in the bearing and the vacuum atmosphere. A plurality of helicalseal grooves extend along the seal area. An annular groove formed on theouter surface of the rotatable cylinder extends along a boundary areadefined between the bearing area and the seal area. An aperture extendsthrough the fixed cylinder at a location opposed to the annular groove.The motor drives and rotates the rotatable cylinder. The rotation causesthe air outside the fixed cylinder to pass through the aperture and intothe clearance (especially to the region between the bearing area and theopposed area of the fixed cylinder). This forms a pressure gas film, orthe compressed gas layer, which radially supports the rotary shaft.

[0007] When the rotating speed of the rotatable cylinder is lower than apredetermined value, the rotatable cylinder slides on the fixedcylinder. Ceramics having relatively high anti-wear properties, such asalumina and zirconia, may be used as the material of the fixed androtatable cylinders.

[0008] When designing the turbo-molecular pump, the depressurizingcapability of the motor determines the number of vanes and the motorspeed. For example, the motor speed is 50,000 rpm to 70,000 rpm in atypical, compact turbo-molecular pump.

[0009] The viscous friction produced by air increases the temperature ofthe air bearing during high speed rotation. The generated heat istransferred rather easily from the outer surface of the fixed cylinder.On the other hand, since the rotatable cylinder is covered by the fixedcylinder, heat cannot be transferred from the rotatable/cylinder soeasily. This results in a large difference between the temperature ofthe fixed cylinder and the temperature of the rotatable cylinder. Thecoefficient of thermal expansion for ceramics, such as alumina andzirconia, is 7 to 8×10⁻⁶/° C. and thus relatively high. Therefore, in anair bearing made of alumina or zirconia, the temperature differencebetween the fixed cylinder and the rotatable cylinder causes thedimension change of the fixed cylinder to differ from that of therotatable cylinder. This varies the size of the clearance. Consequently,the rotatable cylinder may contact the fixed cylinder and obstruct highspeed rotation.

[0010] A fan is often used to cool the air bearing. The fan is effectivefor cooling the outer part of the air bearing, or the fixed cylinder,but not for cooling the inner part, or the rotatable cylinder. Hence,the fan further increases the temperature difference between therotatable cylinder and the fixed cylinder and changes the size of theclearance. There is a demand for a more compact turbo-molecular pumpthat operates at higher rotating speeds. In such a pump, the size of theclearance must be decreased. Therefore, the effects of heat on the airbearing cannot be ignored.

[0011] To increase the speed of the motor, a bearing that has animproved seal and improved performance is necessary. The vibrations ofthe rotatable cylinder affect the supporting characteristics of therotatable cylinder. For example, the depth of the dynamic pressuregrooves affect the natural frequency of the rotatable cylinder. When thenatural frequency (Hz) of the rotatable cylinder and the rotating speed(rps) of the rotatable cylinder are about the same, the possibility ofresonance is high. Resonance causes vibrations of the motor. Therefore,the depth of the dynamic pressure grooves is an important factor forobtaining improved bearing characteristics. Further, the depth of theseal grooves affects the seal characteristics. Hence, the depth of theseal grooves is an important factor for obtaining a high degree ofvacuum.

SUMMARY OF THE INVENTION

[0012] It is an object of the present invention to provide a compactmotor applicable to high speeds and a compact turbo-molecular pumphaving a high depressurizing capability.

[0013] To achieve the above object, the present invention provides amotor including a rotary shaft and a bearing for radially supporting therotary shaft. The bearing includes a cylindrical rotary member connectedto the rotary shaft, and a cylindrical fixed surface surrounding therotary member. The fixed surface is spaced from the rotary member by apredetermined distance. The material of the rotary member has acoefficient of thermal expansion that is smaller than that of thematerial of the fixed surface.

[0014] Another aspect of the present invention provides a motorincluding a rotary shaft and a bearing for radially supporting therotary shaft, wherein the bearing includes a cylindrical rotary memberconnected to the rotary shaft and a cylindrical fixed surfacesurrounding the rotary member. The fixed surface is spaced from therotary member by a predetermined distance. The rotary member is made ofa material having a coefficient of thermal expansion that is 5×10⁻⁶/° C.or less.

[0015] A further aspect of the present invention provides aturbo-molecular pump including a housing, a stator vane attached to thehousing, a rotor vane rotated relative to the stator vane, and a motorfor driving the rotor vane. The motor includes a rotary shaft and abearing for radially supporting the rotary shaft. The bearing includes acylindrical rotary member connected to the rotary shaft and acylindrical fixed surface surrounding the rotary member. The fixedsurface is spaced from the rotary member by a predetermined distance.The material of the rotary member has a coefficient of thermal expansionthat is smaller than that of the material of the fixed surface.

[0016] A further aspect of the present invention provides aturbo-molecular pump including a housing, a stator vane attached to thehousing, a rotor vane rotated relative to the stator vane, and a motorfor driving the rotor vane. The motor includes a rotary shaft and abearing for radially supporting the rotary shaft. The bearing includes acylindrical rotary member connected to the rotary shaft and acylindrical fixed surface surrounding the rotary member. The fixedsurface is spaced from the rotary member by a predetermined distance.The rotary member is made of a material having a coefficient of thermalexpansion that is 5×10⁻⁶/° C. or less.

[0017] A further aspect of the present invention provides a motorincluding a rotary shaft and a bearing for radially supporting therotary shaft. The bearing includes a cylindrical rotary member connectedto the rotary shaft and a cylindrical fixed surface surrounding therotary member. The fixed surface is spaced from the rotary member by apredetermined distance. At least one of the rotary member and the fixedsurface has a dynamic pressure groove formed on a predetermined firstarea defined on a surface opposing the other of the rotary member andthe fixed surface. At least one of the rotary member and the fixedsurface has a seal groove formed on a predetermined second area definedon a surface opposing the other one of the rotary member and the fixedsurface. The seal groove is formed deeper than the dynamic pressuregroove.

[0018] A further aspect of the present invention provides aturbo-molecular pump including a housing, a stator vane attached to thehousing, a rotor vane rotated relative to the stator vane, and a motorfor driving the rotor vane. The motor includes a rotary shaft and abearing for radially supporting the rotary shaft. The bearing includes acylindrical rotary member connected to the rotary shaft and acylindrical fixed surface surrounding the rotary member. The fixedsurface is spaced from the rotary member by a predetermined distance. Atleast one of the rotary member and the fixed surface has a dynamicpressure groove defined on a surface opposing the other of the rotarymember and the fixed surface. At least one of the rotary member and thefixed surface has a first seal groove formed on a surface opposing theother of the rotary member and the fixed surface. The first seal grooveis formed deeper than the dynamic pressure groove.

[0019] Other aspects and advantages of the present invention will becomeapparent from the following description, taken in conjunction with theaccompanying drawings, illustrating by way of example the principles ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The invention, together with objects and advantages thereof, maybest be understood by reference to the following description of thepresently preferred embodiments together with the accompanying drawingsin which:

[0021]FIG. 1 is a cross-sectional view showing a turbo-molecular pumpaccording to a first embodiment of the present invention;

[0022]FIG. 2 is a cross-sectional view showing a brushless motor of theturbo-molecular pump of FIG. 1;

[0023]FIG. 3 is a side view, partly in cross-section, showing an airbearing of the brushless motor of FIG. 2;

[0024]FIG. 4 is a graph showing the relationship between the size of theair bearing clearance and the temperature difference between a rotarycylinder and a fixed surface;

[0025]FIG. 5 is a graph showing the relationship between the size of anair bearing clearance and the temperature difference between a rotarycylinder and a fixed surface in a turbo-molecular pump according to asecond embodiment of the present invention;

[0026]FIG. 6A is a side view, partly in cross-section, showing an airbearing of a turbo-molecular pump according to a third embodiment of thepresent invention;

[0027]FIG. 6B is a diagram showing the depths of the grooves formed inthe rotary cylinder of FIG. 6A;

[0028]FIG. 7 is a graph showing the relationship between the seal groovedepth and the groove inlet pressure; and

[0029]FIG. 8 is a graph showing the relationship between the dynamicpressure groove depth and the natural frequency of the rotary cylinder.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] A turbo-molecular pump 1 according to a first embodiment of thepresent invention will now be described with reference to FIGS. 1 to 4.

[0031] As shown in FIG. 1, the turbo-molecular pump 1 has a housing 2and a brushless motor 10. The housing 2 includes an upper housing 3, alower housing 4, and an annular support block 5. The support block 5 isheld between the upper and lower housings 3, 4. The upper housing 3, thesupport block 5, and the lower housing 4 are fastened together by aplurality of bolts 6.

[0032] An intake 3 a, which is connected to a vacuum chamber (notshown), extends through the top end of the upper housing 3. Gas in thevacuum chamber is drawn into the motor 10 through the intake 3 a. Aplurality of supports 8 are attached to the inner wall of the upperhousing 3. The supports 8 support a plurality of stator vanes 7, each ofwhich extends inwardly from the upper housing 3. An annular supportadapter 9 is fixed to the lower surface of the support block 5.

[0033] O-rings 11, 12 are fitted in the inner surfaces of the supportblock 5 and the support adapter 9, respectively. The motor 10 issupported by the support block 5 and the support adapter 9. The O-rings11, 12 seal the space between the motor 10, the support block 5, and thesupport adapter 9. An intake region 18 is defined above the supportblock 5. A discharge region 19 is defined below the support block 5 andthe support adapter 9.

[0034] The motor 10 has a rotary shaft 13, the upper end of whichextends into the intake 3 a. A cup-like wheel 14 is fastened to theupper end of the rotary shaft 13 by a nut 15. A plurality of rotor vanes16 extend from the peripheral surface of the wheel 14. The rotor vanes16 are each arranged in a gap formed between an associated pair ofstator vanes 7. The motor 10 includes a motor case 17, which is spacedslightly from the inner cylindrical surface of the wheel 14.

[0035] An outer helical groove 17 a extends along the outer surface ofthe motor case 17 and is opposed to the inner surface of the wheel 14. Aplurality of air cooling slits 17 b extend through part of the outersurface of the motor case 17 in the discharge region 19.

[0036] An air cooling fan unit 20 is attached to the lower housing 4.The fan unit 20 has a motor 21 and a fan 22, which is connected to themotor 21. The fan unit 20 produces an air current directed toward theslits 17 b.

[0037] The structure of the brushless motor 10 will now be described.

[0038] As shown in FIG. 2, a pair of annular plugs 23, 24 are fittedinto the ends of the motor case 17. An upper bore 23 a extends throughthe center of the upper plug 23, and a lower bore 24 a extends throughthe center of the lower plug 24. The bores 23 a, 24 a receive the rotaryshaft 13.

[0039] A rotary element 25, which is fixed to the shaft 13, rotatesintegrally with the shaft 13. The rotary element 25 includes a fieldmagnet 26, a pair of bushings 27 a, 27 b, a rotary tube, or cylinder 28,and a pair of rotary magnets 39, 40. The field magnet 26 has fourpermanent magnet plates (not shown). The four permanent magnet platesare parallel to one another and extend axially about the shaft 13.Further, the four permanent magnet plates are arranged to form acylindrical shape. The polarity of each magnet plate differs from thatof the adjacent magnet plates in an alternating manner. The rotarycylinder 28, or rotary member, covers the field magnet 26. The twobushings 27 a, 27 b seal the space between the rotary cylinder 28 andthe shaft 13 and hold the field magnet 26 in between. The bushings 27 a,27 b function to adjust the rotating balance of the rotary element 25.

[0040] A cylindrical fixed tube, or surface 29 is secured to the motorcase 17 to cover the rotary element 25. The fixed surface 29 and theshaft 13 are coaxial. The inner surface of the fixed surface 29 isground to improve its friction characteristics. The rotary cylinder 28and the fixed surface 29 form an air bearing 30 that radially supportsthe rotary shaft 13.

[0041] Three equally spaced armature coils 31 are arranged about theperipheral surface of the fixed surface 29. The angular dimension ofeach armature coil 31 is within a range of 90° to 120°. A cylindricalyoke 32 is secured to the inner surface of the motor case 17 to surroundthe armature coils 31.

[0042] Three magnetic sensors (Hall devices) 33 are arranged along theouter surface of the fixed surface 29 in correspondence with the threearmature coils 31. The three armature coils 31 and the three magneticsensors 33 are each connected to a plurality of connector pins 34 (onlyone shown in FIG. 2), which extend through the plug 24. The connectorpins 34 are electrically connected to an external control circuit (notshown). The magnetic sensors 33 detect changes in the polarity of thefield magnet 26. The control circuit detects the rotating speed based onthe detection signal of the magnetic sensors 33 and controls the currentprovided to the armature coils 31 accordingly to control the rotatingspeed of the shaft 13.

[0043] A pair of magnetic bearings 37, 38 support the rotary shaft 13 ina non-contacting manner. The upper magnetic bearing 37 includes theupper rotary magnet 39 and an upper fixed magnet 41, which is fixed tothe upper plug 23. The lower magnetic bearing 38 includes the lowerrotary magnet 40 and a lower fixed magnet 42, which is fixed to thelower plug 24. The facing surfaces of the magnets 39, 41 have the samepolarity. Thus, the magnets 39, 41 repel one another. Further, thefacing surfaces of the magnets 40, 42 have the same polarity and themagnets 40, 42 repel one another. The magnets 39-42 are all made of thesame material, preferably from a neodymium magnet or a samarium magnet.

[0044] The rotary cylinder 28 and the fixed surface 29 are coaxial. Aclearance having a predetermined radial distance C is provided betweenthe rotary cylinder 28 and the fixed surface 29. The clearance distanceC is predetermined in accordance with the capability of the motor 10(e.g., the motor rotating speed). If the rotating speed of the motor 10is in a range of 60,000 rpm to 90,000 rpm, the clearance distance C is10 μm or lower. If the rotating speed of the motor 10 is in a range of50,000 rpm to 70,000 rpm, the clearance distance C is 3 μm to 6 μm. Inthe first embodiment, the clearance distance C is set to 5 μm.

[0045] The rotary cylinder 28 and the fixed surface 29 are made ofsintered ceramic. More specifically, the rotary cylinder 28 is made ofsilicon carbide, and the fixed surface 29 is made of alumina. Thecoefficient of thermal expansion of silicon carbide is 3 to 4×10⁻⁶/° C.The coefficient of thermal expansion of alumina is 7 to 8×10⁻⁶/° C. Inother words, the coefficient of thermal expansion of the material usedfor the fixed surface 29 is greater than that of the material used forthe rotary cylinder 28.

[0046] As shown in FIG. 3, an upper air bearing area 43 a, a lower airbearing area 43 b, and a gas seal area 44 are defined on the outersurface of the rotary cylinder 28. The gas seal area 44 is locatedcloser to the upper end of the rotary cylinder 28 than the upper airbearing area 43 a. A plurality of equally spaced V-shaped grooves(dynamic pressure grooves) 45 a, 45 b, which are arranged in aherringbone pattern, extend circumferentially along the upper and lowerair bearing areas 43 a, 43 b, respectively. A helical groove (sealgroove) 46 extends along the gas seal area 44. An annular groove 47 aextends between the upper air bearing area 43 a and the gas seal area44, and an annular groove 47 b extends between the upper and lower airbearing areas 43 a, 43 b.

[0047] Referring to FIGS. 2 and 3, a plurality of apertures 48 extendthrough the fixed surface 29 at locations corresponding to the annulargrooves 47 a, 48 a. When the motor 10 is driven, the V-shaped grooves 45a, 45 b function to draw the air outside the fixed surface 29 throughthe apertures 48 and toward the outer surface of the rotary cylinder 28.As the rotating speed of the rotary element 25 increases, the amount ofair drawn into the clearance between the rotary cylinder 28 and thefixed surface 29 increases thereby forming compressed gas layers. Thecompressed gas layers prevent the rotary element 25 from contacting thefixed surface 29 during rotation.

[0048] As the motor 10 drives the rotary shaft 13 and rotates the wheel14, the air in the intake 3 a is drawn into the space between the rotorvanes 16 and the stator vanes 7. The air is drawn into the outer helicalgroove 17 a of the motor case 17 and into the space between the rotaryshaft 13 and the motor case 17. Then, the air flows through the innerhelical groove 46 and the apertures 48 into a gap 36 formed between thefixed surface 29 and the motor case 17. The air is then released intothe discharge region 19 through the slits 17 b. The released air isdischarged through vent holes 4 a formed in the lower housing 4. As theair passes through the outer helical groove 17 a and the inner helicalgroove 46, the velocity of the air increases significantly. The velocitychange increases the fluid drawing effect. The outer helical groove 17 aand the inner helical groove 46 extend in a direction corresponding tothe rotation direction. Thus, the flow of air in the grooves 17 a, 46 isrestricted to one direction. This prevents reversed air flow in thegrooves 17 a, 46 and increases the depressurization capability of thepump 1.

[0049] When the motor 10 is driven, the V-shaped grooves 45 a, 45 bfunction to form a high-pressure compressed gas layer at the axiallymiddle portion of each air bearing area 43 a, 43 b. The compressed gaslayer causes the rotary element 25 to float away from the inner surfaceof the fixed surface 29 when the rotating speed approaches 5000 rpm. Inthis state, the rotary shaft 13 (the rotary element 25) is radiallysupported by the air bearing 30.

[0050] When a certain amount of time elapses after the motor 10 reachesa normal rotating speed, the viscous friction of air heats the rotarycylinder 28 and the fixed surface 29. In this state, the air currentproduced by the fan unit 20 passes through the slits 17 b and toward thefixed surface 29, which cools the fixed surface 29 with forced air.Hence, the temperature increase of the fixed surface 29 is relativelysmall. The rotary cylinder 28 is not cooled with forced air. Hence, thetemperature increase of the rotary cylinder 28 is relatively large.Accordingly, when the pump 1 is operated, the temperature differencebetween the rotary cylinder 28 and the fixed surface 29 reachesapproximately 80° C. to 100° C.

[0051] However, the coefficient of thermal expansion of the rotarycylinder 28 is smaller than that of the fixed surface 29. Accordingly,the change in the outer diameter of the rotary cylinder 28 is about thesame or slightly greater than that of the inner diameter of the fixedsurface 29. Thus, the temperature-related change in the distance Cbetween the fixed surface 29 and the rotary cylinder 28 is small.

[0052]FIG. 4 is a graph showing the relationship between the clearancedistance C (μm) and the temperature difference ΔT (° C.) between therotary cylinder 28 and the fixed surface 29. In the graph, the blacksquares represent points in the relationship when the rotary cylinder 28is made of silicon carbide and the fixed surface 29 is made of alumina,as in the first embodiment. The black diamonds represent points in therelationship when the rotary cylinder 28 and the fixed surface 29 areboth made of alumina. The relationship indicated by the black diamondsis referred to as a comparative example. The maximum temperaturedifference between the rotary cylinder 28 and the fixed surface 29 inthe operation range is represented by ΔTmax. The initial clearancedistance C is, for example, 3 to 6 μm. If the rotary shaft 13 is rotatedat 50,000 to 70,000 rpm, ΔTmax is about 80 to 120° C.

[0053] It is apparent from the graph of FIG. 4 that the decrease in theclearance distance C, which is caused by changes in the temperature, issmaller when the coefficient of thermal expansion of the material of therotary cylinder 28 is smaller than that of the fixed surface 29.

[0054] The coefficient of thermal expansion of the rotary cylinder andthe fixed surface in the comparative example is 7 to 8×10⁻⁶/° C. Thecoefficient of thermal expansion of the rotary cylinder is thusrelatively large, and the difference between the coefficient of thermalexpansion of the rotary cylinder and that of the fixed surface is null.Therefore, in the comparative example, the clearance is eliminatedbefore the temperature difference ΔT reaches the maximum temperaturedifference ΔTmax.

[0055] In comparison, in the first embodiment, the coefficient ofthermal expansion of the material of the rotary cylinder 28 is 3 to4×10⁻⁶/° C., whereas the coefficient of thermal expansion of thematerial of the fixed surface 29 is 7 to 8×10⁻⁶/° C. The coefficient ofthermal expansion of the rotary cylinder 28 is relatively small, and thedifference between the two coefficients of thermal expansion is about4×10⁻⁶/° C. Thus, the reduction of the clearance distance C caused bychanges in the temperature is smaller than that of the comparativeexample. As a result, the clearance is not eliminated when thetemperature difference ΔT reaches the maximum temperature differenceΔTmax. Accordingly, contact between the rotary cylinder 28 and the fixedsurface 29 is prevented regardless of continuous high speeds, and afurther increase in the rotating speed is possible even if the initialvalue of the clearance distance C is 3-6 μm.

[0056] The desired effect is achieved as long as a ceramic having a lowcoefficient of thermal expansion of 5×10⁻⁶/° C. or less is used as thematerial of the rotary cylinder. The desired effect is guaranteed totake place when the coefficient of thermal expansion is 4×10⁻⁶/° C. orless. As long as the difference in the coefficients of thermal expansionbetween the rotary cylinder 28 and the fixed surface 29 is 1×10⁻⁶/° C.or more, the desired effect is obtained. The desired effect isguaranteed to take place when the difference between the coefficients ofthermal expansion is 2×10⁻⁶/° C. or more.

[0057] For example, since the coefficient of thermal expansion ofzirconia is 7 to 8×10⁻⁶/° C. and close to that of alumina, zirconia maybe used as the material of the fixed surface 29.

[0058] The coefficient of thermal expansion of silicon nitride is 3 to4×10⁻⁶/° C. and close to that of silicon carbide. Thus, silicon nitridemay be used as the material of the rotary cylinder 28.

[0059] By using silicon carbide or silicon nitride as the material ofthe rotary cylinder 28, heat dissipation of the rotary cylinder 28 isimproved since the coefficient of thermal conductivity of the rotarycylinder 28 is relatively high.

[0060] The first embodiment has the advantages described below.

[0061] The rotary cylinder 28 is formed from a ceramic material having acoefficient of thermal expansion that is smaller than that of the fixedsurface 29. Thus, the clearance may be set at a relatively small valueof several micrometers. Accordingly, the motor 10 and theturbo-molecular pump 1 can be made smaller and can be operated at higherspeeds.

[0062] Since the clearance is small, the turbo-molecular pump 1 achievesa higher degree of vacuum at high rotating speeds.

[0063] An oxide ceramic such as alumina or zirconia is used as thematerial of the fixed surface 29. Thus, the machining and manufacturingof the air bearing 30 is relatively simple, which reduces cost.

[0064] The rotary cylinder 28 is made of silicon carbide or siliconnitride. Thus, the coefficient of thermal expansion of the rotarycylinder 28 is small and heat is easily dissipated. This reduces thetemperature of the rotary cylinder 28 and limits the reduction of theclearance distance C. Accordingly, the motor 10 can be operated athigher rotating speeds.

[0065] The O-rings 11, 12, which are made of rubber, elastically connectthe motor 10 to the housing 2. This absorbs the vibrations of the motor10 that would otherwise be transmitted to the housing 2.

[0066] A turbo-molecular pump according to the present invention willnow be described. In the second embodiment, the material of the rotarycylinder 28 differs from that of the fixed surface 29. The rotarycylinder 28 is made of silicon carbide, and the fixed surface 29 is madeof silicon nitride. The coefficient of thermal expansion of the rotarycylinder 28 is 3 to 4×10⁻⁶/° C., which is about the same as that of thefixed surface 29.

[0067]FIG. 5 is a graph showing the relationship between the clearancedistance C (μm) and the temperature difference ΔT (° C.) between therotary cylinder 28 and the fixed surface 29. The black circles representpoints in the relationship when the rotary cylinder 28 is made ofsilicon carbide and the fixed surface 29 is made of silicon nitride. Inother words, the circles represent the second embodiment. As shown inFIG. 5, the reduction in the clearance distance C that is caused bychanges in the temperature is smaller than that of the comparativeexample. In the second embodiment, the difference between thecoefficients of thermal expansion of the materials used for the rotarycylinder 28 and the fixed surface 29 is substantially null. However, thecoefficient of thermal expansion of the rotary cylinder 28 is 3 to4×10⁻⁶/° C. and relatively small. Thus, the clearance is not eliminatedwhen the temperature difference ΔT reaches the maximum temperaturedifference ΔTmax. Accordingly, contact between the rotary cylinder 28and the fixed surface 29 is prevented at continuous high speeds and afurther increase in the rotating speed is possible.

[0068] In the second embodiment, the material combination of the rotarycylinder 28 and the fixed surface 29 may be changed to silicon carbideand silicon carbide, silicon nitride and silicon nitride, or siliconnitride and silicon carbide, respectively. These material combinationsalso achieve the necessary clearance and permit high speed rotationwithout contact between the rotary cylinder 28 and the fixed surface 29.

[0069] When the coefficients of thermal expansion of the rotary cylinder28 and the fixed surface 29 are substantially the same, the desiredeffect is achieved if the material of the rotary cylinder 28 has acoefficient of thermal expansion of 5×10⁻⁶/° C. or lower. The desiredeffect is further guaranteed if the material has a coefficient ofthermal expansion of 4×10⁻⁶/° C. or lower.

[0070] The materials used for the rotary cylinder 28 and the fixedsurface 29 are carbides or nitrides, which have coefficients of thermalconductivity that are higher than those of oxides. The rotary cylinder28 and the fixed surface 29 thus have improved heat dissipation andavoid contact.

[0071] A turbo-molecular pump according to a third embodiment of thepresent invention will now be described with reference to FIGS. 6A to 8.

[0072] As shown in FIG. 6B, like the first embodiment, there areV-shaped grooves 45 a, 45 b, a helical groove 46, and annular grooves 47a, 47 b in the rotary cylinder 28. The depth of each type of groovediffers from that of the other types. FIG. 7 is a graph showing therelationship between the depth of the helical groove 46 and the pressureat the inlet of the groove (Pa). The groove inlet pressure is thepressure that acts on the upper end (right side as viewed in FIG. 6A) ofthe helical groove 46. It is preferred that the value of the grooveinlet pressure be as small as possible. More specifically, it ispreferred that the groove inlet pressure be 10² Pa or lower. The lowerend of the helical groove 46 is exposed to the compressed air layer.Thus, it is required that the helical groove 46 (gas seal area 44)function as an air seal under a pressure difference of 10² Pa or more.

[0073] As apparent from FIG. 7, the seal effect of the gas seal area 44depends on the depth of the helical groove 46. The desired seal isobtained when the depth of the helical groove 46 is two to ten times theclearance distance C. To obtain a high sealing effect, the preferreddepth of the helical groove 46 is four to eight times the clearancedistance C.

[0074] The relationship between the depth of the V-shaped grooves 45 a,45 b and the natural frequency of the rotary element 25 will now bedescribed. The motor 10 is used within a range of about 60,000 to 90,000rpm. Accordingly, the rotating speed, or frequency, of the rotaryelement 25 during operation is 1,000 to 1,500 rps (Hz). To avoidresonance, the value of the natural frequency of the rotary element 25(in Hz) must differ greatly from the operation speed (in RPS).

[0075] As apparent from FIG. 8, the natural frequency of the rotaryelement 25 decreases as the depth of the V-shaped grooves 45 a, 45 bincreases. When the depth of the V-shaped grooves 45 a, 45 b is one tofive times the clearance distance C, the natural frequency is notincluded in the operation speed range. If the depth of the V-shapedgrooves 45 a, 45 b is 6C, the natural frequency of the rotary element 25approaches the operation speed range (about 1000 to 1500 Hz). In suchstate, resonance is apt to occur and smooth rotation of the rotary shaft13 may be hindered. Accordingly, the preferred depth of the V-shapedgrooves 45 a, 45 b is one to five times the clearance distance C.

[0076] To smoothly draw air through the apertures 48 and into theclearance, the depth of the annular grooves 47 a, 47 b must not exceed avalue that is two times the depth of the V-shaped grooves 45.

[0077] The depth of the annular grooves 47 a, 47 b is required to exceedtwo times the depth of the V-shaped grooves 45 a, 45 b. Thus, tofacilitate manufacture, it is preferred that the clearance distance C bethree to fifteen times the depth of the V-shaped grooves 45 a, 45 b. Themanufacture of the rotary cylinder 28 is especially facilitated when thedepth of the annular grooves 47 a, 47 b is equal to the sum of the depthof the V-shaped grooves 45 a, 45 b and the depth of the helical groove46.

[0078] When the depth of the V-shaped grooves 45 a, 45 b is representedby A_(PS), the depth of the annular grooves 47 a, 47 b by A_(R), and thedepth of the helical groove 46 by A_(GS), the range of each value isA_(PS)=1×C to 5×C, A_(GS)=2×C to 10×C, and A_(R)=3×C to 15×C. It isespecially effective if A_(GS) equals 4×C to 8×C. It is preferred thatthe depth combination satisfy the relationship of A_(R)=A_(PS)+A_(GS).The preferred clearance distance C is about ten micrometers or less.

[0079] The third embodiment has the advantages described below.

[0080] When the motor 10 is driven, the V-shaped grooves 45 a, 45 bfunction to form a high-pressure compressed gas layer with the air drawnin through the apertures 48 at the axially middle portion of each airbearing area 43 a, 43 b. The depth of the annular grooves 47 is set atA_(R)=3C to 15C. During rotation, air flows smoothly from the apertures48 to the clearance. Thus, the compressed gas layer is readily formed.When the rotating speed approaches 5,000 rpm, the compressed gas layerscause the rotary element 25 to float away from the inner surface of thefixed surface 29. That is, the rotary element 25 is supported by the airbearing 30.

[0081] The depth of the V-shaped grooves 45 a, 45 b is set at A_(PS)=1Cto 5C. Thus, the natural frequency of the rotary element 25 differsgreatly from the operation speed range and prevents the rotary element25 from resonating. Accordingly, satisfactory bearing characteristicsare obtained. Further, the rotary shaft 13 is stable in the operationspeed range.

[0082] The depth of the helical grooves 46 is set at A_(GS)=2C to 10C.Thus, a high vacuum degree of 10² Pa or lower is guaranteed. If thecondition of A_(GS)=4C to 8C is satisfied, a vacuum degree of 10 Pa orlower is obtained.

[0083] The outer helical groove 17 a increases the degree of vacuum(about 1 Pa or less). In addition, the relative rotation between therotor vanes 16 and the stator vanes 7 functions to further decrease thepressure of the intake region thereby achieving an ultra-high vacuum. Inother words, the degree of vacuum generated by the turbo-molecular pump1 is determined not only by the depressurization capability of the rotorvanes 16 and the stator vanes 7 but also by the seal characteristics ofthe outer helical groove 17 a and the gas seal area 44. In the thirdembodiment, the seal characteristics of the helical groove 46 and theouter helical groove 17 a are improved. Thus, the turbo-molecular pump 1produces a greater vacuum.

[0084] The depth of the annular grooves 47 a, 47 b is set equal to thesum of the depth of the V-shaped grooves 45 a, 45 b and the depth of thehelical groove 46 (A_(R)=A_(PS)+A_(GS)). Thus, three types of grooves 45a-47 b having different depths may be formed through two groove formingprocesses.

[0085] It should be apparent to those skilled in the art that thepresent invention may be embodied in many other specific forms withoutdeparting from the spirit or scope of the invention. Particularly, itshould be understood that the present invention may be embodied in thefollowing forms.

[0086] In the first embodiment, any kind of material may be used as longas the coefficient of thermal expansion of the material of the rotarycylinder 28 is smaller than that of the material of the fixed surface29. For example, the rotary cylinder 28 may be made of alumina and thefixed surface 29 may be made of zirconia. Further, boron nitride oraluminum nitride may be used as the material of the rotary cylinder 28or the fixed surface 29.

[0087] In the first embodiment, the ceramic material of the rotarycylinder 28 and the ceramic material of the fixed surface 29 is notlimited to materials in which the difference in the coefficients ofthermal expansion is equal to or greater than a predetermined value. Aslong as the coefficient of thermal expansion of the material of therotary cylinder 28 is 5×10⁻⁶/° C. or lower, a material combinationhaving a small difference in the coefficients of thermal expansion alsomaintains the clearance of the air bearing 30 in the operation speedrange.

[0088] In the third embodiment, due to the anti-wear characteristics andheat characteristics, ceramic is preferred as the material for therotary cylinder 28 and the fixed surface 29. However, at least one ofthe rotary cylinder 28 and the fixed surface 29 may be made of plasticas long as the material has the necessary anti-wear and heatcharacteristics. Further, the material of the rotary cylinder 28 and thematerial of the fixed surface 29 may either be the same or different.

[0089] In the first and second embodiments, a ceramic oxide, such asmullite or zircon, which resist wear, may be used as the material of thefixed surface 29.

[0090] In the first and second embodiments, cordierite, which is anoxide having a low coefficient of thermal expansion, may be used as thematerial of the rotary cylinder 28.

[0091] In the first and second embodiments, a static air bearing may beemployed as the air bearing 30.

[0092] In the first and second embodiments, an air bearing may beemployed in lieu of the magnetic bearings 37, 38, which support therotary shaft.

[0093] In the first and second embodiments, the outer helical groove 17a may be eliminated.

[0094] In the third embodiment, the depth of the annular grooves 47 a,47 b is not limited to the sum of the depth of the V-shaped grooves 45a, 45 b and the depth of the helical groove 46. As long as the annulargrooves 47 a, 47 b have a depth that is two times the depth of theV-shaped grooves 45 a, 45 b, a sufficient amount of air is provided tothe clearance and a high-pressure compressed gas layer is formed.

[0095] In the third embodiment, the depth of the annular grooves 47 a,47 b may be changed as required. However, if the depths of the V-shapedgrooves 45 a, 45 b and the helical groove 46 is set in the same manneras the third embodiment, the seal and bearing characteristics areimproved.

[0096] In the first to third embodiments, at least one of the dynamicpressure grooves 45 a, 45 b and the helical groove 46 may be formed onthe inner surface of the fixed surface 29.

[0097] In the first to third embodiments, the motor 10 may be applied toan apparatus other than the turbo-molecular pump 1, for example, acompressor.

[0098] In the first to third embodiments, the helical grooves 17 a, 46extend in a direction corresponding to the rotating direction of themotor 10. However, if, for example, the motor 10 is employed in acompressor, the helical grooves 17 a, 46 may extend in the oppositedirection.

[0099] The present examples and embodiments are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein, but may be modified within the scope andequivalence of the appended claims.

What is claimed is:
 1. A motor comprising: a rotary shaft; and a bearingfor radially supporting the rotary shaft, wherein the bearing includes:a cylindrical rotary member connected to the rotary shaft; and acylindrical fixed surface surrounding the rotary member, wherein thefixed surface is spaced from the rotary member by a predetermineddistance, and wherein the material of the rotary member has acoefficient of thermal expansion that is smaller than that of thematerial of the fixed surface.
 2. The motor according to claim 1,wherein the difference between the coefficient of thermal expansion ofthe material of the fixed surface and the coefficient of thermalexpansion of the material of the rotary member is 1×10⁻⁶/° C. or more.3. The motor according to claim 1, wherein the material of the rotarymember has a coefficient of thermal expansion that is 5×10⁻⁶/° C. orless.
 4. The motor according to claim 3, wherein the fixed surface ismade of a ceramic oxide material.
 5. The motor according to claim 4,wherein the fixed surface is made of alumina or zirconia.
 6. The motoraccording to claim 3, wherein the rotary member is made of a ceramiccarbide material or a ceramic nitride material.
 7. The motor accordingto claim 6, wherein the rotary member is made of silicon carbide orsilicon nitride.
 8. The motor according to claim 1, further comprising acase for accommodating the bearing, the rotary member, and the fixedsurface, wherein the case has a slit.
 9. A motor comprising: a rotaryshaft; and a bearing for radially supporting the rotary shaft, whereinthe bearing includes: a cylindrical rotary member connected to therotary shaft; and a cylindrical fixed surface surrounding the rotarymember, wherein the fixed surface is spaced from the rotary member by apredetermined distance, and wherein the rotary member is made of amaterial having a coefficient of thermal expansion that is 5×10⁻⁶/° C.or less.
 10. The motor according to claim 9, wherein the rotary memberis made of a ceramic carbide material or a ceramic nitride material. 11.The motor according to claim 10, wherein the rotary member is made ofsilicon carbide or silicon nitride.
 12. The motor according to claim 9,further comprising a case for accommodating the bearing, the rotarymember, and the fixed surface, wherein the case has a slit.
 13. Aturbo-molecular pump comprising: a housing; a stator vane attached tothe housing; a rotor vane rotated relative to the stator vane; and amotor for driving the rotor vane, wherein the motor includes: a rotaryshaft; and a bearing for radially supporting the rotary shaft, whereinthe bearing includes: a cylindrical rotary member connected to therotary shaft; and a cylindrical fixed surface surrounding the rotarymember, wherein the fixed surface is spaced from the rotary member by apredetermined distance, and wherein the material of the rotary memberhas a coefficient of thermal expansion that is smaller than that of thematerial of the fixed surface.
 14. The pump according to claim 13,further comprising a device for cooling the motor.
 15. A turbo-molecularpump comprising: a housing; a stator vane attached to the housing; arotor vane rotated relative to the stator vane; and a motor for drivingthe rotor vane, wherein the motor includes: a rotary shaft; and abearing for radially supporting the rotary shaft, wherein the bearingincludes: a cylindrical rotary member connected to the rotary shaft; anda cylindrical fixed surface surrounding the rotary member, wherein thefixed surface is spaced from the rotary member by a predetermineddistance, and wherein the rotary member is made of a material having acoefficient of thermal expansion that is 5×10⁻⁶/° C. or less.
 16. Thepump according to claim 15, further comprising a device for cooling themotor.
 17. A motor comprising: a rotary shaft; and a bearing forradially supporting the rotary shaft, wherein the bearing includes: acylindrical rotary member connected to the rotary shaft; and acylindrical fixed surface surrounding the rotary member, wherein thefixed surface is spaced from the rotary member by a predetermineddistance, wherein at least one of the rotary member and the fixedsurface has a dynamic pressure groove formed on a predetermined firstarea defined on a surface opposing the other of the rotary member andthe fixed surface, and wherein at least one of the rotary member and thefixed surface has a seal groove formed on a predetermined second areadefined on a surface opposing the other one of the rotary member and thefixed surface, the seal groove being formed deeper than the dynamicpressure groove.
 18. The motor according to claim 17, wherein the depthof the seal groove is within a range of two to ten times thepredetermined distance.
 19. The motor according to claim 18, wherein thedepth of the dynamic pressure groove is within a range of one to fivetimes the predetermined distance.
 20. The motor according to claim 17,wherein at least one of the rotary member and the fixed surface has anannular groove formed between the first predetermined area and thesecond predetermined area, wherein the annular groove is deeper than theseal groove.
 21. The motor according to claim 20, wherein the depth ofthe annular groove is within a range of three to fifteen times thepredetermined distance.
 22. The motor according to claim 20, wherein thedepth of the annular groove is substantially equal to the depth of theseal groove and the depth of the dynamic pressure groove.
 23. The motoraccording to claim 17, wherein the seal groove is helical.
 24. Aturbo-molecular pump comprising: a housing; a stator vane attached tothe housing; a rotor vane rotated relative to the stator vane; and amotor for driving the rotor vane, wherein the motor includes: a rotaryshaft; and a bearing for radially supporting the rotary shaft, whereinthe bearing includes: a cylindrical rotary member connected to therotary shaft; and a cylindrical fixed surface surrounding the rotarymember, wherein the fixed surface is spaced from the rotary member by apredetermined distance, wherein at least one of the rotary member andthe fixed surface has a dynamic pressure groove defined on a surfaceopposing the other of the rotary member and the fixed surface, andwherein at least one of the rotary member and the fixed surface has afirst seal groove formed on a surface opposing the other of the rotarymember and the fixed surface, the first seal groove being formed deeperthan the dynamic pressure groove.
 25. The pump according to claim 24,wherein the motor includes a generally cylindrical case, wherein thepump further comprises a cup-like wheel coupled to a distal end of therotary shaft to cover the case and support the rotor vane, the wheelhaving an inner cylindrical surface that is separated from an outercylindrical of the case during operation of the motor, and wherein atleast one of the wheel and the case has a second seal groove formed on asurface opposing the other of the wheel and the case.
 26. The pumpaccording to claim 25, wherein the second seal groove is helical. 27.The pump according to claim 24, wherein the motor is elasticallysupported by the housing via an elastic member.